How are underwater tunnels built?

Some underwater tunnels are made from giant tubes that are built on land, then such in a deep trench dug in the river bottom.

It is sometimes better to tunnel under a river than to build a bridge across it.

Building an underwater tunnel is more difficult than boring a tunnel through a mountain. In solid rock, a tunnel holds its shape. In the mud and sand of a river bottom the problem is to keep the water out and the sides from caving in.

In building an underwater tunnel, men work in a large, water tight chamber that holds back the mud and water while the tunnel is being built.

A barrel-shaped shields, as big as the tunnel, is pushed forward through the mud by powerful jacks, as workmen inside the shield dig away the sand and mud and build up the lining of the tunnel. To hold the water out, compressed air is pumped into the working room.

Sometimes, underwater tunnels are made by sinking giant steel tubes the size of the tunnel into a trench that has been dug across the river bottom.